1. Field of the Invention
The invention relates to an underlayment sheet of the type applied in overlapping courses to a roof deck for a building structure, and more particularly toward such an underlayment sheet of the self-adhering type used to enhance water and weather resistance capabilities of the roof system.
2. Related Art
In typical pitched roof applications for residential and commercial buildings, a water-resistant membrane in the form of an underlayment sheet is installed between the roof deck and the outer roof covering. In some applications, the underlayment may even comprise the final roofing layer without any shingles or other coverings applied over top. The underlayment sheet is typically applied in overlapping courses and is designed to help the roof shed water and provide secondary weather protection.
In applications where water issues are a particular concern, either from ice damming or low-slope roof pitches, or severe weather encounters, various special underlayment techniques have been proposed. More traditionally, a two-ply underlayment system known as the 30/90 hot mop system is used. In this system, a base sheet of 30 lb traditional underlayment material is affixed to a plywood or OSB roof decking with so-called “tin tag” or “cap nail” fasteners to provide mechanical attachment directly to the decking. The 30 lb underlayment sheets are applied in overlapping courses beginning at the bottom (eaves) edge and working in parallel rows toward the ridge. After the 30 lb underlayment sheet has been installed, a roofing contractor applies hot asphalt over the entire surface. The asphalt may be heated to 525° F., typically, at which temperature the hot, runny asphalt poses a significant burn issue to workers on and around the roof. A 90 lb underlayment material is then applied directly over the hot, unset asphalt to establish a bonded two-ply system. This traditional method usually requires two trips to the job site and different crews to apply first the 30 lb underlayment sheet in overlapping courses and then later the hot asphalt with 90 lb top layer. Additionally, different equipment is used for the different phases. A roof may not be considered water tight until the complete two-ply system has been installed.
A more recent underlayment system has been proposed for establishing a water-proof underlayment, which is known generally as the peel and stick method. Originally intended for applications in which ice damming was a concern, the peel and stick underlayment products have found increased acceptance in warmer client markets to help shed roof water such as may be encountered in storm conditions, and which may be more problematic in low-slope situations where water accumulation tends to be more of an issue. Typically, the peel and stick underlayment is designed to be installed sticky side down directly to the plywood decking in overlapping courses. Some in the industry suggest that such peel and stick products cause less harm to the roof deck than traditional underlayment systems because they are less susceptible to corrosion. In addition, they are not installed with nails or screws so they do not weaken the deck or transfer heat to the interior which could make air conditioning systems less efficient. Also, there are no holes that could allow moisture to seep in. Accordingly, while the peel and stick products have been touted as an effective improvement over the traditional two-ply 30/90 hot mop system, many local building codes have opposed use of the peel and stick underlayment directly upon the plywood decking. Many municipalities are requiring a base sheet of traditional (organic felt or fiberglass) underlayment sheet anchored with nails to the roof decking prior to applying the peel and stick products over top. Code bodies reason that when it is time for a re-roof, the peel and stick applied directly to the roof decking will cause delamination damage to the plywood or OSB decking. Delamination of the decking could lead to replacement of the plywood decking, thus driving the cost of future re-roofs up significantly. If used in this code-approved fashion, the peel and stick underlayment is hardly more economical than the traditional 30/90 hot mot two-ply system.
Accordingly, there is a need for an improved waterproof or water-resistant underlayment system for roof structures which is less expensive than the current peel and stick products used according to code recommendations, and less complicated and dangerous to apply than the 30/90 hot mop two-ply systems. Furthermore, there is a need to provide such a waterproof underlayment system which is not likely to cause delamination damage to plywood/OSB decking, thereby facilitating future re-roof projects. Furthermore, there is a need for a waterproof underlayment system which uses fewer nails or screws than current methods, thereby increasing the strength of the deck and diminishing heat transfer to the building interior.